When percolation occurs in conventional carburetors and the pressure of fuel atomized in the float chamber exceeds a certain level, atomized fuel is emitted from the air vent into the air passage, whereby the fuel ratio in the fuel-air mixture supplied to the engine becomes higher. Particularly, when new fuel is fed to the float chamber which is at a high temperature, the new fuel boils suddenly and the atomized fuel produced thereby in the float chamber jets out through the air vent into the air passage, thereby suddenly increasing the ratio of fuel in the fuel-air mixture.
Since, as described above, the fuel ratio of the fuel-air mixture pulsatingly increases or decreases because the atomized fuel jets out into the air passage each time the pressure thereof in the float chamber exceeds a certain level, conventional carburetors thus cause the engine to operate poorly, a condition which is commonly called rough idling.
This invention obviates the above-mentioned disadvantage by providing a carburetor having a further passage which provides continuous communication between the float chamber and the intake pipe. This further passage, which communicates with the intake pipe downstream of the throttle valve, at least when the throttle valve is closed or is only partially open, thus minimizes undesired fluctuations in the fuel-air mixture as supplied to the engine.